The present invention relates to a system for battery monitoring in motor vehicle applications and more particularly to a battery condition monitoring system for diesel engine equipped vehicles.
Lead acid batteries are the conventional source for power used by automatic starters to crank internal combustion engines installed on motor vehicles. Lead acid batteries also provide auxiliary power for other electrical components installed on such vehicles. Failure of a battery to supply power for starting can necessitate jump starting the engine or an expensive and time consuming call to service for assistance. It would be an advantage to operators to receive warning of impending battery failure in time to take corrective action before failure of a battery in the field.
Apparent battery failure can stem from a number of causes. U.S. Pat. No. 5,281,919 to Palanisamy lists a number of possible failure modes relating to start failure of an engine. These include: a low state of charge of the battery; loss of battery capacity (leading to a low state of charge); corroded battery terminals; low electrolyte levels in the cells; a defective charging system; and a defective starting system. Low temperatures magnify these problems by reducing the current supporting capacity of the battery and increasing the power requirements to crank an engine. The most common failure mode for a battery are the progressive increase in the battery""s internal resistance and subsequent loss of battery cranking and charge capacity.
Lead acid batteries operate chemically. The chemical reactions that produce current during discharge are not perfectly reversible during recharge. A battery discharges several hundred ampere-seconds during cranking of an engine. Recharging then occurs during the first few minutes after the engine begins running. The cycle of repeated discharge and subsequent recharge of lead acid batteries results in chemical imbalances in and loss of the electrolyte solution, the formation of undesirable compounds on battery plates and physical deterioration of the plates. Lead acid batteries are constructed from closely spaced, alternating plates of sponge lead (Pb), which serve as the negative plates, and lead dioxide (PbO2), which serve as the positive plates. The plates are preferably substantially immersed in a sulfuric acid (H2SO4) water solution, which serves as an electrolyte. During battery use, lead sulfate (PbSO4) forms on both the negative and positive plates. The concentration of acid in the electrolyte decreases. As the plates become more chemically similar and the acid strength of the electrolyte falls, a battery""s voltage will begin to fall. From fully charged to fully discharged each cell loses about 0.2 volts in potential (from about 2.1 volts to 1.9 volts).
Optimally, recharging of a battery reverses the process, strengthening the acid in the electrolyte and restoring the original chemical makeup of the plates. However, recharging can have other effects, including polarization of the battery, overheating and the electrolytic decomposition of the water into molecular hydrogen and oxygen. Occurrence of these factors results in the battery not returning to its original state. Electrolysis of the water reduces the physical volume, and quantity, of the electrolyte. Electrolytic breakdown of the water leaves the electrolyte excessively acidic, with consequential degradation of the battery plates. High temperatures developed during recharging can promote sulfation of the battery plates (i.e. the formation of hardened, relatively insoluble crystalline lead sulfate on the surface of the plates), which in turn increases a battery""s internal resistance. To some extent sulfation and other factors resulting in the slow reduction of a lead acid battery""s charge capacity can be controlled by avoiding overcharging, or by avoiding overheating of the battery stemming from excessively fast recharging. Polarization results in a poorly mixed electrolyte and a condition where battery voltage reflects a full 2.1 volts per cell, but only because local areas of the electrolyte contain over concentrations of acid, which in turn can damage the plates. As the physical condition of a battery deteriorates, its capacity to hold a charge, in terms of ampere-hours declines. This is the case even though the battery continues to exhibit a 2.1 volt potential per cell when charged to maximum. Accordingly, battery cranking power is not accurately reflected by open circuit potential voltage.
Battery condition is best indicated by the specific gravity of the battery""s electrolytic solution. Conventionally, the best way to gauge the state of charge of a lead acid battery has been to measure the specific gravity of the electrolyte of a properly filled (and exercised) battery using a temperature compensated hydrometer. A load test of the battery under controlled conditions may be used, either in conjunction with a check of specific gravity or independently. A load test subjects a fully charged battery to an ampere load equal to xc2xd the rated cold cranking capacity of the battery (at xe2x88x9218xc2x0 C.) for 15 seconds, then measures the voltage and the current under load and requires referral to a voltage chart to assess battery condition. See page 48, Storage Battery Technical Service Manual, Tenth Edition, published by the Battery Council International, Chicago, Ill. (1987). Such procedures are obviously not easily practiced in the field, where driver/operators of vehicles could make use of a quick indication if a battery has sufficient cranking power to start an engine.
To meet the need for battery condition evaluation in the field, the prior art has proposed numerous battery condition monitoring systems, which rely on other indirect indications of battery condition. In broad overview, a lead-acid battery will exhibit different operating characteristics when new as opposed to when used. As the battery deteriorates it will exhibit a higher internal resistance, and will not accept as great a current input. Voltage under load will fall off more rapidly. Indicators related to these factors may be monitored to give an indication of battery condition. However, difficulties arise from the inability to control the conditions of the evaluation. One such system directed to determining battery condition is U.S. Pat. No. 5,744,963 to Arai et al. Arai et al. teach a battery residual capacity estimation system Residual capacity is estimated from a current integration method which utilizes a voltage-current trend calculating section, sensors for obtaining battery current and terminal voltage, a voltage-current straight line calculating section, and a comparator operation for detecting when residual capacity has declined compared to a prior period residual capacity.
Palanisamy, U.S. Pat. No. 5,281,919, describes a method of monitoring a vehicle battery used with a gasoline engine. Five variables are monitored including ambient temperature (T), battery voltage (V), power source (typically an alternator/voltage regulator) voltage (Vs), battery current (I) and time (t). From these variables, the patent provides algorithms for determining the battery""s State of Charge (SOC), internal resistance (IR), polarization (PR), and performs various diagnostics.
Palanisamy determines the battery""s SOC using a combination of charge integration and open circuit voltage measurements. The open circuit portion of the test relies on a 0.2 voltage drop per cell from a fully charged lead acid cell to a discharged lead acid cell. Open circuit battery voltage (OCV or VOCV) may be taken with the engine on, but is measured at point in time which avoids effects of polarization of the battery. Open circuit voltage is deemed to coincide with the absence of current flows into or out of the battery for a minimum period (i.e. I greater than  greater than 0 for a minimum period T). Current integration counts current flow (I) into and out of the battery. Monitoring starts from a point of predetermined charge of the battery, preferably a full charge as determined by the open circuit voltage test. As Palanisamy observes, current integration is subject to error from battery outgassing and deterioration of the physical condition of the battery. The combination of the results is offered as an accurate measurement of the battery""s state of charge, but, due to the systematic errors identified in the patent, is not an necessarily an accurate measurement of the battery""s condition.
Internal resistance (IR) is estimated from the open circuit voltage, the first measured voltage from the power source and the current flow from the battery following imposition of the starting load. Power output capacity is estimated from IR. Battery polarization arises from non-uniformity of electrolyte density between battery plates and is estimated using Vs, Is, the last battery voltage reading during starting. IR can be used to get battery output capacity for a variety at various temperatures, and then used for a comparison to a table of engine start power requirements supplied by the engine manufacturer.
Palanisamy is limited due to the fact that, under common operating conditions, the current required to crank a gasoline engine is substantially less than the load requirements of a standard load test. Cranking of a gasoline engine usually does not generate data of anywhere near the quality of data produced by controlled condition load test making reference to published voltage charts useless as a mechanism for determining battery conditional.
The invention provides a method and an apparatus for monitoring a vehicle battery installed on a motor vehicle and indicating the condition thereof to a vehicle operator without necessitating removal of the battery from the vehicle. Upon movement of a vehicle ignition switch from off to on, a process of evaluating the vehicle battery is started. The process comprises steps for measuring an open circuit voltage for the vehicle battery and the ambient temperature. The open circuit voltage is compared to a table of allowable open circuit voltage ranges as a function of ambient temperature to determine, as an initial matter, if the open circuit voltage is within acceptable ranges for the battery as indicated by manufacturer""s specifications. If the open circuit voltage falls within the acceptable range, it is determined if sufficient time has passed since the most recent execution of the routine to avoid polarization effects on the measured open circuit voltage.
If the possibility of polarization effects on the measured open circuit voltage is indicated by a brief lapse since the vehicle battery was last exercised, a load test is imposed on the vehicle battery by engaging an engine starter system to crank the vehicle engine. If the test is automated a safety interlock may be provided based, for example, on whether the hood is open or closed. After a period T, which is preferably fixed in advance, of cranking the engine, voltage across the terminals of the vehicle battery and current from the vehicle battery are measured, both events occurring while the battery remains under the load imposed by cranking. A specification table indicating battery capacity as a function of the results of the load test, possibly updated by battery history, provides a value indicating battery capacity indicated by the results of the load test. An engine required cranking power specification using engine sensor measurements as inputs provides a value for comparison to the capacity figure. A result of the test provides an input criterion for generating a displayable result.
Additional effects, features and advantages will be apparent in the written description that follows.